cmd: Handle `if exist` with an empty string argument.

programs/cmd/builtins.c

@@ -2841,13 +2841,16 @@ int evaluate_if_condition(WCHAR *p, WCHAR **command, int *test, int *negate)
     WCHAR *param = WCMD_parameter(p, 1+(*negate), NULL, FALSE, FALSE);
     int    len = lstrlenW(param);
 
-    if (!len) goto syntax_err;
-    /* FindFirstFile does not like a directory path ending in '\' or '/', append a '.' */
-    if (param[len-1] == '\\' || param[len-1] == '/') lstrcatW(param, L".");
+    if (!len) {
+        *test = FALSE;
+    } else {
+        /* FindFirstFile does not like a directory path ending in '\' or '/', so append a '.' */
+        if (param[len-1] == '\\' || param[len-1] == '/') wcscat(param, L".");
 
-    hff = FindFirstFileW(param, &fd);
-    *test = (hff != INVALID_HANDLE_VALUE );
-    if (*test) FindClose(hff);
+        hff = FindFirstFileW(param, &fd);
+        *test = (hff != INVALID_HANDLE_VALUE);
+        if (*test) FindClose(hff);
+    }
 
     WCMD_parameter(p, 2+(*negate), command, FALSE, FALSE);
   }

programs/cmd/tests/test_builtins.cmd

@@ -1176,6 +1176,11 @@ if exist "subdir/" (
 ) else (
    echo ERROR exist subdir with / and quotes not working
 )
+if not exist "" (
+    echo exist empty string works
+) else (
+    echo exist empty string broken
+)
 del foo subdir\bar
 rd subdir
 

programs/cmd/tests/test_builtins.cmd.exp

@@ -833,6 +833,7 @@ exist subdir with \ and quotes ok
 exist subdir with /. ok
 exist subdir with / ok
 exist subdir with / and quotes ok
+exist empty string works
 ------ for numbers
 negative numbers handled
 negative numbers handled